1. Field of the Invention
The present invention relates generally to the production of polyolefin, and more particularly to an improved measure of flow rate of the polymer slurry circulating in a polyolefin loop reactor.
2. Description of the Related Art
This section is intended to introduce the reader to aspects of art that may be related to aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
As chemical and petrochemical technologies have advanced, the products of these technologies have become increasingly prevalent in society. In particular, as techniques for bonding simple molecular building blocks into longer chains (or polymers) have advanced, the polymer products, typically in the form of various plastics, have been increasingly incorporated into various everyday items. For example, polyolefin polymers, such as polyethylene, polypropylene, and their copolymers, are used for retail and pharmaceutical packaging, food and beverage packaging (such as juice and soda bottles), household containers (such as pails and boxes), household items (such as appliances, furniture, carpeting, and toys), automobile components, pipes, conduits, and various industrial products.
Specific types of polyolefins, such as high-density polyethylene (HDPE), have particular applications in the manufacture of blow-molded and injection-molded goods, such as food and beverage containers, film, and plastic pipe. Other types of polyolefins, such as low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), isotactic polypropylene (iPP), and syndiotactic polypropylene (sPP) are also suited for similar applications. The mechanical requirements of the application, such as tensile strength and density, and/or the chemical requirements, such thermal stability, molecular weight, and chemical reactivity, typically determine what type of polyolefin is suitable.
One benefit of polyolefin construction, as may be deduced from the list of uses above, is that it is generally non-reactive with goods or products with which it is in contact. This allows polyolefin products to be used in residential, commercial, and industrial contexts, including food and beverage storage and transportation, consumer electronics, agriculture, shipping, and vehicular construction. The wide variety of residential, commercial and industrial uses for polyolefins has translated into a substantial demand for raw polyolefin which can be extruded, injected, blown or otherwise formed into a final consumable product or component.
To satisfy this demand, various processes exist by which olefins may be polymerized to form polyolefins. Typically, these processes are performed or near at petrochemical facilities, which have ready access to the short-chain olefin molecules (monomers and comonomers) such as ethylene, propylene, butene, pentene, hexene, octene, decene, and other building blocks of the much longer polyolefin polymers. These monomers and comonomers may be polymerized in a liquid-phase polymerization reactor and/or gas-phase polymerization reactor to form a product comprising polymer (polyolefin) solid particulates, typically called fluff or granules. The fluff may possess one or more melt, physical, rheological, and/or mechanical properties of interest, such as density, melt index (MI), melt flow rate (MFR), copolymer content, comonomer content, modulus, and crystallinity. The reaction conditions within the reactor, such as temperature, pressure, chemical concentrations, polymer production rate, and so forth, may be selected to achieve the desired fluff properties.
In addition to the one or more olefin monomers, a catalyst for facilitating the polymerization of the monomers may be added to the reactor. For example, the catalyst may be a particle added via a reactor feed stream and, once added, suspended in the fluid medium within the reactor. An example of such a catalyst is a chromium oxide containing hexavalent chromium on a silica support. Further, a diluent may be introduced into the reactor. The diluent may be an inert hydrocarbon, such as isobutane, propane, n-butane, n-pentane, i-pentane, neopentane, and n-hexane, which is liquid at reaction conditions. However, some polymerization processes may not employ a separate diluent, such as in the case of selected examples of polypropylene production where the propylene monomer itself acts as the diluent.
The discharge of the reactor typically includes the polymer fluff as well as non-polymer components, such as unreacted olefin monomer (and comonomer), diluent, and so forth. In the case of polyethylene production, the non-polymer components typically comprise primarily diluent, such as isobutane, having a small amount of unreacted ethylene (e.g., 5 wt. %). This discharge stream is generally processed, such as by a diluent/monomer recovery system, to separate the non-polymer components from the polymer fluff. The recovered diluent, unreacted monomer, and other non-polymer components from the recovery system may be treated, such as by treatment beds and/or a fractionation system, and ultimately returned as purified or treated feed to the reactor. Some of the components may be flared or returned to the supplier, such as to an olefin manufacturing plant or petroleum refinery. As for the recovered polymer (solids), the polymer may be treated to deactivate residual catalyst, remove entrained hydrocarbons, dry the polymer, and pelletize the polymer in an extruder, and so forth, before the polymer is sent to customer.
In an industry where billions of pounds of polyolefin product are produced per year, small incremental improvements, for example, in catalyst activity, monomer yield, energy efficiency, diluent recovery, increased throughput, and so forth, can generate significant cost savings in the manufacture of polyolefins. Fortunately, technological advances over the years in raw materials, equipment design and operation, and the like, have provided great strides in reducing the capital, operating, and fixed costs of polyolefin manufacturing systems. However, the competitive business of polyolefin production drives manufacturers to continuously improve their processes in order to increase production, lower production costs, and so on. One need in the manufacture of polyethylene is to reduce the demand for olefin-free diluent (which is used in catalyst preparation, flushes where catalyst may be present, etc.). Indeed, processing of recycled diluent to produce olefin-free diluent can be capital intensive and consume extensive energy.
Moreover, in order to understand the performance of the loop reactor, as well the design and operation of the loop reactor, it is beneficial to have reliable measurements of the flow rate and pressure of the polyolefin slurry circulating in the loop reactor. An accurate measurement of flow rate may provide, for example, a better understanding of properties of the slurry, the actual throughput of polymer in the reactor, feedstock and energy consumption, and so on. Further, a consistent flow measurement of the circulating slurry may facilitate operating a greater throughput or production rate because of being able to operate at a higher production rate closer to the fouling curve of the loop reactor or to the slurry flow solids carrying limit without solids settling in the loop reactor, for example.